Strip foundations are the most popular in private construction. It is ideal for the construction of small houses, garages, bathhouses and other outbuildings. All construction work can be done with your own hands, and the relatively low consumption of materials and the minimum amount of excavation work can reduce the price and production time. Of course, for everything to go as expected, you need to know how to properly reinforce the foundation.
Before telling you how to properly reinforce a strip foundation, it’s worth saying a few words about the choice of reinforcement.
- If you need to reinforce the foundation for a one-story or two-story house, as well as lighter buildings, you should take reinforcement with a diameter of 10-24 millimeters. Thicker material will cost too much and its high strength will not be used. Less thick reinforcement may not withstand the load.
- It is advisable to use special corrugated fittings. It provides a better connection to concrete, guaranteeing its high strength and reliability. The smooth analogue costs a little less, but is not suitable for use due to low adhesion. The only exception is cross connections. The load on them is significantly less.
- If the soil is homogeneous over the entire area of the foundation, then you can use material with a cross-section of 10-14 millimeters. With heterogeneous soil, the load on the base increases, so it is advisable to invest in rods with a diameter of 16-24 millimeters.
Of course, buying thick corrugated fittings is quite expensive. But if you decide to reinforce the strip foundation with your own hands, then the amount of work is not too large. So, you will have to overpay a maximum of a few hundred rubles - this is fully compensated by the high durability and reliability of the finished structure.
When independently calculating and selecting reinforcement for the reinforcement frame of a strip foundation, there is a high probability of error. In the future, it may cause the destruction of the house, so the best solution would be to order a foundation reinforcement project from a designer, and carry out the binding of the frame yourself according to the drawing.
How much reinforcement is needed?
Before you go to the store to buy material, you need to find out how much it will be needed to reinforce the strip foundation. To do this, you should think in advance which reinforcement scheme for a strip foundation will be the best choice and carry out calculations for a specific object.
An example of frame reinforcement for a foundation
When building small houses, garages and bathhouses, the following frame configuration is usually used:
- 2 belts: upper and lower;
- each belt consists of 3-4 reinforcement rods;
- the optimal distance between the rods is 10 centimeters. Please note that the distance from the reinforcement to the edges of the future foundation should be at least 5 centimeters;
- The connection of the belts is carried out using clamps or pieces of reinforcement at a pitch of 5-30 centimeters, depending on the area of reinforcement.
This scheme is optimal. Now, knowing the dimensions of the future building, it is not at all difficult to carry out the appropriate calculations.
Let's say you want to build a spacious frame or wooden cottage with an area of 150 square meters with external walls with a perimeter of 50 meters. Let's carry out calculations based on this. When reinforcing the strip foundation, we use SNiP and the characteristics described above.
We have two belts with three rods in each. Total - 6 multiplied by 50 = 300 meters of main reinforcement. We take into account the number of jumpers, which are laid in increments of 30 centimeters. To do this, divide 50 meters by 0.3. We get 167 pieces. The transverse jumpers in this base will have a length of 30 centimeters, and the vertical ones - 60 centimeters. For vertical jumpers you will need 167x0.6x2=200.4 meters. On horizontal ones - 167x0.3x2=100.2 meters. In total, 300 meters of thick corrugated reinforcement and 300.6 meters of thinner, smooth reinforcement will be required. Having received these numbers, you can safely go to the store for material - a strip foundation without reinforcement will not last long. Some experts recommend taking reinforcement with a margin of 10-15%. After all, a certain amount of material will be needed to strengthen the corner parts of the strip foundation and will be used for joining.
How to knit the frame?
The rules for reinforcing strip foundations force one to abandon the use of welding in favor of knitting, since when welding is used, in the places of welded joints, metal rods lose strength up to 2-2.5 times. In addition, this is where corrosion most often appears, which can damage the reinforcement within a few years, significantly reducing the reliability and durability of the foundation. Only connection by knitting is permissible. This is a rather complicated stage, which will take a lot of time for an inexperienced user. However, a lot here depends on what tool you use.
Reliable knot for tying reinforcement with wire
The classic tool for tying reinforcement in strip foundations is a special crochet hook. Using it, experienced craftsmen can produce up to 12-15 knots per minute (of course, if the knitting wire is prepared and cut in advance). The main advantage of this option is its affordability - a hook can be bought in many stores for a hundred rubles or even cheaper. The downside is that the speed of working with it is not great even for experts. Please note that you will have to make many hundreds of ties even if you have to reinforce small foundations.
Wire and hook for knitting the frame
If you want to finish the job quickly, you can use a special knitting gun. Working with it, even an inexperienced user can easily produce 25-30 knots per minute. That is, productivity will increase at least 2 times. Alas, the cost of such equipment is not low - from 50 thousand and above. In addition, to work with it you need a special wire - a regular one may not be suitable. This further increases the cost. But if you have the opportunity to rent a knitting gun for a few hours or a day, feel free to agree to such an offer, just don’t forget to find out the maximum diameter of the reinforcement that it can tie. Working with high-quality tools, you will spend a maximum of a day assembling the frame - correct reinforcement of the strip foundation becomes much easier and faster. When working manually, this process can take a week or more.
How to make a frame correctly?
Before proceeding with the reinforcement of the strip foundation, drawings of suitable frames should be studied. After all, the strength of the frame determines whether the foundation will serve for many decades or will become covered with cracks in the first spring due to seasonal fluctuations in soil level.
In order not to make mistakes during manufacturing, you need to remember a few rules:
- The overlap (the distance from the place of knitting to the edge of the rod) should be at least 5 centimeters.
- At corner joints, perpendicularly running rods must be connected to each other - in no case should you use two separate blocks that are not connected to each other. The ideal solution would be corners made from bent reinforcement - this type of foundation reinforcement scheme is the most reliable. But for this you need to have special equipment; if the reinforcement has a diameter of 14 millimeters or more, smaller diameters can be bent at home.
- Connections using wire must be tight - if you use a crochet hook, then tighten the wire until it stops so that there is no free space between the clamp and the main reinforcement. Also check with your hand; if the clamp moves when touched, you should make an additional tie with wire.
- The overlap during reinforcement should be equal to 40-50 diameters of the reinforcement. There must be a separation between adjacent connecting rods and the top and bottom layers, according to the design.
- The reinforcing frame must stand level in the formwork. It is also necessary to take care of a protective layer of concrete for the reinforcement, done according to the requirements of the drawing. It should be remembered that the minimum protective layer is equal to the diameter of the reinforcement.
Bending of all elements for foundation reinforcement is performed cold. Do not heat the reinforcement under any circumstances, as this will lead to a loss of its strength.
As you can see, the rules are as simple as possible. But some inexperienced builders are unaware or forget about their existence. This leads to the fact that the reinforcement technology of the strip foundation is disrupted and its service life is significantly reduced.
Excavation and preparatory work
One of the advantages of a strip foundation is the relatively small amount of excavation work. A couple of people, working every day with short breaks, can easily dig a pit of a suitable size on normal soil. When the foundation pit is ready, you can begin to arrange it.
The first step is making the foundation cushion. Thanks to it, the negative impact of groundwater on the foundation is reduced, and the load from the foundation itself and the entire building is distributed as evenly as possible over the ground. Here you can use different materials. Most often sand or crushed stone is used. They do their job well - the main thing is that the thickness of the pillow is at least 15-20 centimeters.
But some experts recommend a concrete pad. Yes, it costs the most. Expensive cement and the need to reinforce the cushion greatly increase the cost and time of construction. But as a result, you get the most reliable foundation for the foundation, guaranteeing that it will last for many years. Therefore, we can say with confidence that this money will not be thrown away.
An example of a reinforced strip foundation
If work is carried out on weak, heaving soil, or if you plan to build a heavy brick house, but the use of a monolithic foundation is undesirable for some reason, then you can use a strip foundation with a sole. The widening (glass) can significantly reduce the load on the ground. Of course, do not forget about the reinforcement of the foundation shell - on heaving soils it will regularly withstand significant tensile and bending loads. It is very important to provide it with sufficient strength.
When using a foundation with a sole, the volume of excavation work increases. In addition, you will have to spend additional money on reinforcing the base of the strip foundation - if it fails, this will lead to the rapid destruction of the entire structure.
Formwork is installed on top of the finished cushion. When choosing the width, keep in mind that the finished foundation should be 10-15 centimeters thicker than the external load-bearing walls.
The next step is to install waterproofing. Some builders use roofing felt, but this is a rather expensive material. And the heavy weight makes the installation process difficult. Therefore, it is quite possible to use construction polyethylene. Yes, it is less durable. But it is only needed for a few days - so that the cement laitance does not go into the sand. Therefore, cheap and lightweight polyethylene is quite suitable. It is laid on top of the formwork. At the joints, make a larger overlap - at least 10-15 centimeters - and glue with wide tape.
This concludes the preparatory work. Now we’ll tell you about pouring and reinforcing the foundation with your own hands.
We install the frame, pour concrete
It is best to assemble a frame from reinforcement directly in a prepared pit - this allows the elements to be most firmly fixed. But if we are talking about reinforcing a buried strip foundation or if the pit is too narrow for work to be carried out directly in it, then you can assemble the frame from the outside above the trench, and then carefully lower it into place. Problems usually do not arise here and step-by-step instructions are not needed.
The last and one of the most critical stages is pouring the foundation.
Pouring a strip foundation with a concrete pump
It is advisable to use concrete grade M200 or higher for this. It has high strength to withstand significant loads, and also has sufficient frost resistance.
It should be said right away that the work will require a large amount of material. Carry out all the necessary calculations in advance - you need to pour concrete in one go, avoiding delamination and other separations. Otherwise, the strength of the foundation will significantly decrease, and this will affect the safety of the house. For the same reason, it is advisable to rent a concrete mixer. Today many companies offer such a service. In addition, renting cheap models is relatively inexpensive - less than a thousand rubles per day. If you work intensively during this time, it is quite possible to cope with the work. In addition, the presence of a concrete mixer allows you to increase productivity - you just need to throw in sand, cement and pour in water, soon getting a finished product that just needs to be poured onto the frame installed in the formwork. Working with shovels, such productivity cannot be achieved.
After pouring concrete, you must wait 28 days. During this time, the concrete will gain sufficient strength and it will be possible to begin building a house, garage or bathhouse.
We recommend watching a video where an experienced civil engineer will talk about the important nuances of foundation reinforcement. What should you pay attention to when performing work first, so that the foundation of the house is reliable.
Now you know how to reinforce a strip foundation with your own hands. To do this, it is not at all necessary to have highly specialized skills or buy expensive equipment. It is enough to know, at least in theory, how to reinforce a foundation. Experience will come with the process, and all tools can be replaced with cheap analogues or rented, saving money and time.
The foundation is the basis of any building. The quality of operation of the house and its durability will depend on a good foundation. The foundation usually consists of a hardened concrete mixture. To impart rigidity to the concrete base, it must be reinforced with special steel rods according to certain rules.
There is a scheme for reinforcing a strip foundation, following which the builder will lay a strong and durable foundation for the house.
What is a strip foundation and why reinforce it?
Strip foundation is one of the most common concrete foundations. It is a ribbon structure made around the perimeter of the house, as well as in the areas of partitions.
The strip foundation has the following advantages:
- withstands heavy loads of various buildings made of brick, stone, blocks;
- provides for the arrangement of the basement;
- Suitable for heterogeneous soils where there is a risk of subsidence and swelling.
Thus, the foundation experiences a double load: heavy walls press from above, and soil tension acts from below. The last fact is especially relevant for any building. Indeed, as a result of winter freezing, wet soil increases in volume. If the foundation is not rigid enough, its integrity may be compromised, leading to cracks in the walls and the risk of the house collapsing.
But concrete itself copes well with these loads. So why do we need to add additional steel bars?
This is due to the fact that the load at different points of the foundation is different. After all, the condition of the soil is not the same in different places, just like the pressure of the house, the load on the foundation will be different.
To equalize this physical indicator, steel rods are placed in the concrete base, which ensure uniform distribution of the load over the entire area of the foundation.
Type and quantity of reinforcement to strengthen the foundation
There are 2 types of reinforcing rods: steel and composite. Metal fittings are used more often, as their effectiveness has been proven over the years.
The use of fiberglass is used for those buildings where the requirements for limiting radio interference, magnetic fields, and chemical exposure are increased.
Metal reinforcement can be rod or wire. For a strip foundation, use bar reinforcement of a periodic profile of class A-3 or according to GOST A400. This building material has good adhesion to concrete and the lower and upper parts of the frame are knitted from it.
The vertical and transverse parts of the frame are made from smooth rods with a cross-section of up to 1 cm. Smooth rods are called mounting rods. Transverse reinforcement should be selected class A-1 or according to GOST A240.
Usually the mesh frame is made in 2-3 rows. The step between the vertical bars: 40-70 cm, and the horizontal ones - 30-60 cm. If the buried foundation has a height of less than 1 m, then it will need 2-3 longitudinal levels.
For example, consider a foundation 60 cm high and 30 cm wide. This foundation is laid under a building whose length and width are 5 m each.
In this case, a two-tier mesh is made with a step of 0.5 m. For 4 longitudinal lines of 20 m each, 80 linear meters of working reinforcement will be required. The calculation of mounting vertical rods is taken taking into account indentations from the surface of 5 cm. If the number of intersections = 51, then we get the total length of the rods: 1.4 m * 51 = 71.4 m. It is recommended to buy material with a margin of 10%.
Thus, by adding numbers, we obtain the total amount of required reinforcement: 80 + 71.4 + 10% ~ 170 linear meters.
Video on how to properly reinforce the spatial frame of a shallow strip foundation:
Rules for laying steel rods in a concrete base
Before making a metal frame, the iron rods should be cleaned and their quality checked.
The technology for reinforcing the strip base is performed according to the following algorithm:
- A sand-crushed stone cushion, 5 cm thick, is poured into the dug trench. This is necessary to prevent corrosion of the iron rods.
- Formwork is carried out and a thin layer of concrete is poured.
- Transverse rods are laid on top in increments of 80 cm.
- When forming a frame, longitudinal rods are laid perpendicular to the previous rods in 2 rows. The intersections are connected. The lower level of the frame is ready.
- Vertical smooth rods are installed at the joints. It is important to maintain perpendicularity.
- The upper tier of the frame is attached to the vertical rods. It is a frame in which the rods are fixed at intervals of 20 cm.
- The upper tier is equipped with longitudinal rods, which are fastened to the remaining rods with clamps or wire.
- The reinforcement skeleton is rigidly fixed to the formwork. The gap between the iron structure and the formwork should be 3-5 cm.
- Monitor the quality of fastenings and remove excess debris.
The most important thing when making a frame is to securely fasten the rods together, especially in the corners of the foundation. It is important to maintain even angles and perpendicularity. There are 2 ways to combine rods: welding and wire knitting.
Welding in private construction is undesirable, since this method does not provide the proper quality of the perpendicular structure. Builders often neglect the requirements of the standards and weld manually using resistance welding rather than arc welding.
The preferred method of connecting the rods is knitting with wire, with a diameter of 0.8-3 mm. This is done using a crochet hook. The quality of this connection is much higher than in the previous version. The disadvantages of the method are: the process is more labor intensive and has low rigidity compared to a welded structure.
Figure 1 shows a diagram of strengthening the foundation for a one-story house, measuring 10x6 m.
As longitudinal rods, class A-3 rods with a diameter of 12 mm are used; The transverse rods are reinforcement with a diameter of 8 mm, class A-1.
The spacing of the floors is 0.6 m, and in the area of corners - 0.2 m. Corners and T-shaped intersections are reinforced with haunches - class A-3 reinforcement, with a diameter of 12 mm. In the area of abutments, the haunches are overlapped, which is equal to: 50*d, where d is the diameter of the rod.
Reinforcement of corners and T-shaped joints can be done using special tabs. They are peculiar corners, with the length of the shelves equal to: 50*d, where d is the diameter of the reinforcement. For example, if the diameter of the reinforcement is 10 mm, then the bend of the legs is 500 mm. An example of such fastening is shown in Figure 2.
To summarize, we can highlight the basic rules for foundation reinforcement:
- The diameter of the working rods must be at least 12 mm.
- Longitudinal (working) rods together with transverse reinforcement form a frame, the elements of which are welded or tied.
- For an average size foundation, 3-4 longitudinal rods are needed.
- The diameter of the transverse rods is 6-8 cm, which are laid in increments of 200-600 mm.
- The thickness of the tape base is taken to be at least 300 mm.
- Corners and T-shaped intersections are reinforced with special haunches or paws. The diameter of these fasteners must be equal to the diameter of the working rods.
A metal frame in the foundation design is the key to a strong house and comfortable living in it.
How the French make a strip foundation:
A shallow strip foundation (hereinafter referred to as MZLF) is one of the types of strip foundations, which is characterized by a shallow depth, significantly less than the depth of soil freezing, and a relatively small consumption of concrete mixture. This article discusses the main advantages and disadvantages of MZLF, the most common mistakes in their construction, a simplified calculation method suitable for private developers (not professionals), and recommendations for constructing a foundation with your own hands.
The main advantages of MZLF are:
- economical - concrete consumption is significantly lower than during the construction of a conventional strip foundation. It is this factor that most often determines the choice of this technology in low-rise construction;
- reduced labor costs - less excavation work, less volume of prepared concrete (this is especially important when it is not possible to pour the finished mixture from a mixer);
- smaller tangential forces of frost heaving due to the reduced area of the lateral surface of the foundation.
However, during the construction of the MZLF, it is necessary to strictly adhere to the technology; a frivolous attitude to the process can lead to the appearance of cracks, and then all of the above advantages, as they say, will go down the drain.
The most common mistakes made when installing MZLF:
1) selection of the main working dimensions of the foundation without any (even the most simplified) calculation at all;
2) pouring the foundation directly into the ground without covering it with non-heaving material (sand). According to Fig. 1 (on the right), we can say that in the winter season the soil will freeze to the concrete and, rising, drag the tape upward, i.e. the tangential forces of frost heaving will act on the foundation. This is especially dangerous if the MZLF is not insulated and a high-quality blind area is not equipped;
3) improper reinforcement of the foundation - choosing the diameter of the reinforcement and the number of rods at your discretion;
4) Leaving the MZLF unloaded for the winter - it is recommended to carry out the entire cycle of work (construction of the foundation, erection of walls, and arrangement of the blind area) one construction season before the onset of severe frosts.
Calculation of a shallow strip foundation.
The calculation of the MZLF, like any other foundation, is based, firstly, on the value of the load from the weight of the house itself and, secondly, on the calculated soil resistance. Those. the soil must withstand the weight of the house transmitted to it through the foundation. Please note that it is the soil that supports the weight of the house, and not the foundation, as some believe.
If an ordinary private developer can calculate the weight of a house if desired (for example, using our online calculator located), then it is not possible to determine the calculated soil resistance on your site on your own. This characteristic is calculated by specialized organizations in specialized laboratories after conducting geological and geodetic surveys. Everyone knows that this procedure is not free. Mostly, architects who design a house resort to it, and then, based on the data received, they calculate the foundation.
In this regard, it makes no sense to provide formulas for calculating the size of the MZLF within the framework of this article. We will consider the case when a developer carries out construction on his own, when he does not conduct geological and geodetic surveys and cannot accurately know the calculated soil resistance on his site. In such a situation, the dimensions and design of the MZLF can be selected according to the tables below.
The characteristics of the foundation are determined depending on the material of the walls and ceilings of the house and its number of storeys, as well as on the degree of heaving of the soil. How you can determine the latter is described
I. MZLF on medium and highly heaving soils.
Table 1: Heated buildings with walls made of lightweight brickwork or aerated concrete (foam concrete) and reinforced concrete floors.
Notes:
— the number in brackets indicates the pillow material: 1 — medium-sized sand, 2 — coarse sand, 3 — a mixture of sand (40%) with crushed stone (60%);
— this table can also be used for houses with wooden floors, the safety margin will be even greater;
— see below for foundation design options and reinforcement options.
Table 2: Heated buildings with walls made of insulated wooden panels (frame houses), logs and timber with wooden floors.
Notes:
— the numbers in brackets mean the same as in Table 1;
- above the line value for walls made of insulated wooden panels, below the line - for log and timber walls.
Table 3: Non-buried foundations of unheated log and timber buildings with wooden floors.
Notes:
- above the line values for log walls, below the line - for walls made of timber.
Design options for MZLF on medium- and highly heaving soils, indicated by letters in the tables, are shown in the figures below:
1 — monolithic reinforced concrete foundation; 2 - sand filling of sinuses; 3 — sand (sand-crushed stone) pillow; 4 — reinforcement frame; 5 - blind area; 6 7 — waterproofing; 8 - base; 9 — soil surface; 10 - sand bedding; 11 - turf.
Option a.— the upper plane of the foundation coincides with the surface of the earth, the base is made of brick.
Option b.- the foundation protrudes above the surface by 20-30 cm, forming a low base or being part of the base.
Option c.- the foundation rises 50-70 cm above the ground, while it also serves as a base.
Option d.- non-buried foundation-basement; Table 3 shows that such foundations are used for unheated wooden buildings.
Option d.- used instead of options b. or V. when the width of the base of the foundation significantly exceeds the thickness of the wall (more than 15-20 cm).
Option e.— a shallow strip foundation on a sand backfill is used quite rarely on weak (peaty, silted) soils with a high groundwater level for wooden buildings. Depending on the size of the building, bedding is done either under each strip or under the entire foundation at once.
Reinforcement of a shallow strip foundation.
MZLF reinforcement is made with meshes of working reinforcement and auxiliary reinforcing wire. The working reinforcement is located in the lower and upper parts of the foundation, and it must be immersed in the thickness of the concrete by about 5 cm. The lower mesh works to deflect the foundation tape downward, and the upper mesh works to deflect the tape upward. There is no point in placing the working reinforcement in the middle of the tape (as can sometimes be seen on the Internet).
Table 4: Options for foundation reinforcement.
The MZFL reinforcement schemes are shown in the following figure:
A.— a mesh with two working reinforcement rods; b.— a mesh with three working reinforcement rods; V.— T-shaped joint; G.— L-shaped corner joint; d.— additional MZLF reinforcement with a large sole width, when the sole is more than 60 cm wider than the base (the additional mesh is located only in the lower part.
1 — working fittings (A-III); 2 — auxiliary reinforcing wire ∅ 4-5 mm (Вр-I); 3 — vertical reinforcement rods ∅ 10 mm (A-III), connecting the upper and lower mesh; 4 — reinforcement for strengthening the corner ∅ 10 mm (A-III); 5 — connection with wire strands (twisting length is at least 30 diameters of the working reinforcement); 6 — additional working fittings ∅ 10 mm (A-III).
II. MZLF on non-heaving and slightly heaving soils.
Shallow strip foundations on non-heaving and slightly heaving soils do not have to be made only from monolithic concrete. You can also use other local materials, for example, rubble stone, red ceramic brick. MZLF is laid at 0.3-0.4 meters without a sand cushion. Moreover, for wooden buildings and one-story brick (or aerated concrete) foundations, they don’t even need to be reinforced.
For 2- and 3-story houses with walls made of stone materials, MZLF is reinforced. Concrete foundations are reinforced according to the 1st reinforcement option (see Table 4 above). Foundations made of rubble or brick are reinforced with masonry mesh made from BP-I reinforcement ∅ 4-5 mm with a cell size of 100x100 mm. The nets are placed every 15-20 cm.
MZLF structures on non-heaving and slightly heaving soils are shown in the figure below:
1 - foundation; 2 - base; 3 - blind area; 4 — waterproofing; 5 — subfloor (shown conditionally); 6 - mesh made of wire reinforcement, 7 — reinforcement according to option 1 (see table 4)
Options a. and b.
- for wooden and one-story brick (aerated concrete) buildings. Options c.
and Mr.
— for two- and three-story brick (aerated concrete) buildings.
The width of the sole b is determined depending on the number of storeys of the building and the material of the walls and ceilings.
1) Table 5: Values of the width of the sole of the MZLF on non-heaving and slightly heaving soils.
2) Stages of construction of a shallow strip foundation and recommendations.
3) Before starting construction of the foundation, if necessary, it is necessary to ensure high-quality drainage of surface rainwater from neighboring areas from the building site. This is done by cutting out drainage ditches.
4) A sand (sand-crushed stone) cushion is poured layer by layer (in layers of 10-15 cm) with careful compaction. They use either manual rammers or area vibrators. Tamping should not be taken lightly. Shallow foundations are not as powerful as foundations poured to the full depth of freezing, and therefore freezing here is fraught with the appearance of cracks.
5) The formwork is laid out and the reinforcement frame is knitted. Do not forget to immediately provide water and sewerage to the house. If the foundation is also a plinth, remember about the vents (does not apply to buildings with floors on the ground).
6) Concrete is poured. Filling of the entire tape must be done continuously, as they say, in one go.
7) After the concrete has set (3-5 days in summer), the formwork is removed and made vertical.
8) The sinuses are backfilled with coarse sand with layer-by-layer compaction.
9) A blind area is being constructed. It is advisable (especially with a small height of the foundation tape) to make the blind area insulated. This measure will further reduce the forces of frost heaving affecting the MZLF in winter. Insulation is made with extruded polystyrene foam.
As already mentioned at the beginning of the article, it is not allowed to leave the MZLF unloaded or underloaded (the building is not fully built) for the winter. If this happens, the foundation itself and the soil around it must be covered with any heat-saving material. You can use sawdust, slag, expanded clay, straw, etc. There is also no need to clear the snow on the construction site.
It is highly not recommended to build a shallow strip foundation in frozen soil in the winter.
In the comments to this article, you can discuss with readers your experience in the construction and operation of MZLF or ask questions that interest you.
It is a solid concrete support placed under all load-bearing walls of the house.
The design of such bases is quite simple.
The degree of strength, resistance to emerging loads and load-bearing capacity form the optimal combination that allows the use of a strip foundation in most buildings.
With some additions, this type is capable of serving on different types of soil and in relatively unfavorable geological conditions.
The main element of the structure is the reinforcement frame, which ensures the strength of the tape and resistance to stress.
Concrete is a specific material. It is capable of withstanding significant pressure without visible consequences, but it can withstand multidirectional tensile loads with great difficulty.
A concrete block, which is a monolithic casting without additional reinforcing elements, is capable of withstanding only a uniform compressive load.
If a force is applied in the central part, and the edges of the block are fixed, it will break under a relatively small load.
It is impossible to use it in this form as a base for a construction project.
The problem is solved with the help of a reinforcing frame placed inside the block before casting.
Reinforcement of strip bases is a necessary and mandatory condition prescribed by the requirements of SNiP 52-01-2003. All working aspects of creating reinforced concrete structures are regulated - concrete composition, dimensions and material of reinforcement, type of frame structure, assembly method and other issues.
Compliance with SNiP standards is mandatory for all builders, since only in this way can the reliability of the building and the safety of people be ensured.
How fittings work
Reinforcing bars can carry tensile loads approximately 10 times greater than concrete. Being installed inside the casting, they take on tensile loads, preventing cracks from appearing, strengthening and strengthening the concrete strip.
The reinforcement frame is a spatial lattice consisting of load-bearing and auxiliary rods. If the tape itself in cross-section is a rectangle, then the frame in cross-section forms a similar figure, but somewhat smaller.
If a bending load is applied to the tape, then those rods that are located on the side opposite to the point of application of the force begin to work.
They do not allow the tape to change shape, taking on external influences.
NOTE!
For particularly critical structures, prestressing rods are used, which are tensioned before pouring concrete and released after the mass has hardened. Such foundations are capable of working in difficult conditions, but are not used for private housing construction.
The main elements are horizontal rods - load-bearing, or working. Vertical elements serve to support working reinforcement and in most cases are needed only until the moment of pouring. After this, the working functions are performed only by the corner elements, which experience additional stress and operational loads.
Auxiliary reinforcement is made from thinner rods and is needed to prevent displacement of the main elements during pouring and hardening..
How to choose concrete
- Degree of compressive and axial tensile strength.
- Frost resistance.
- Waterproof.
For residential buildings with low-rise brick or similar buildings, the best option is M300. When using lightweight cellular or porous materials (foam concrete, expanded clay concrete), it is allowed to use less durable and dense concrete - grades M200 and even M150.
More durable varieties are used for critical or multi-story buildings. For example, M400 concrete can be used for casting foundations for residential buildings up to 20 floors high.
Types of fittings
There are two types of fittings:
- Metal.
- Composite.
The first type is the familiar hot-rolled steel rods with a notch that allows for reliable adhesion to concrete. There are rods of different diameters, from 6 to 80 mm, designed for use in appropriate conditions.
For auxiliary reinforcement, both corrugated and smooth rods of smaller diameter can be used.
- A piece of wire is bent in half. The resulting half-loop is inserted under the cross joint of the rods in the diagonal direction.
- The ends of the half-loop are raised up so that the wire wraps around the knot being connected.
- The point of the crochet hook is inserted into the loop, while resting on the other end of the wire. Rotational movements twist the ends, tightly tightening the connected rods.
- The same method is used for knitting longitudinal joints. The only difference is in the position of the wire - it wraps around both connected rods in the transverse, and not in the diagonal direction.
A crochet hook can be purchased at a store, but it’s easier to make it yourself. You need to take a piece of steel wire 405 mm thick, sharpen it slightly and bend one end by about 1.5-2 cm.
For ease of use, the hook can be slightly bent in the middle part. The techniques for working with it are simple, but require some skill, which appears very quickly.
Installation diagram
Reinforcement of a strip foundation is usually done using a metal reinforcement frame assembled by welding or tied with special soft steel wire.
The working rods are installed in a horizontal position in such a way that in cross-section they form a rectangle with sides 10 cm smaller than the width and height of the concrete strip.
This ratio ensures the depth of immersion of the rods in concrete, at which the load-bearing capacity is sufficiently high, but the material is reliably protected from corrosion. Vertical reinforcement serves to fix load-bearing rods in the desired position during concrete hardening.
Both of these processes cause significant loads, so the quality of the reinforcement depends on the strength of the connection.
Photo of the drawing:
Corner reinforcement
The corner elements of the strip foundation, which include T-shaped junctions, are reinforced by installing curved anchors - individual rods bent at the desired angle. Working rods are often bent if their length allows this (for example, at the corners of short walls or abutments).
The corners of the foundation experience increased stress, so the presence of additional anchoring is necessary to increase the strength of the frame connection and increase the load-bearing capacity of this section of the tape.
The main mistakes often encountered when reinforcing corners are:
- Using only the outer contour, with insufficient anchoring of the inner part of the corner.
- Lack of connection between external and internal rods.
- Lack of mechanical connection between the sole and the frame.
- Incorrect placement of rod connection points.
The use of anchors and proper connection with the main elements of the armored belt allows you to avoid mistakes and strengthen critical sections of the frame.
Sole reinforcement
The base of the foundation is the area experiencing maximum heaving loads or lateral pressure from soil water. There are various methods of strengthening the sole, which provide a high-quality connection with the concrete preparatory part, but they are used for the construction of industrial critical structures.
To reinforce the base of the foundation of a low-rise residential building, it is customary to use reinforcing mesh, which increases the strength and immobility of the lower part of the tape. The mesh is mechanically connected to the main frame, this is especially important if it is wider than the tape itself.
If a bending load is applied to the tape, then those rods that are located on the side opposite to the point of application of the force begin to work.
Ready-made or welded meshes with transverse arrangement of rods are used. For areas located on difficult soils, it is recommended to use welded structures from working rods that can withstand loads in all directions.
Useful video
In this video you will learn how to reinforce a strip foundation:
Conclusion
Reinforcement of a strip foundation is the main operation, without which all other work becomes impractical. Seasonal soil movements, changes in groundwater levels, tectonic influences and other influencing factors require the foundation to be strong and able to resist emerging loads.
These qualities can only be provided by a competently and carefully formed armored belt, which forms the internal skeleton of the concrete strip and compensates for all axial tensile loads.
In contact with
The strip foundation has a non-standard geometry: its length is tens of times greater than its depth and width. Due to this design, almost all loads are distributed along the belt. A concrete stone cannot compensate for these loads on its own: its bending strength is not enough. To give a structure increased strength, not just concrete is used, but reinforced concrete - this is a concrete stone with steel elements located inside - steel reinforcement. The process of laying metal is called strip foundation reinforcement. It’s not difficult to do it with your own hands, the calculations are elementary, the diagrams are known.
The quantity, location, diameters and type of reinforcement - all this must be specified in the project. These parameters depend on many factors: both on the geological situation on the site and on the mass of the building being constructed. If you want to have a guaranteed strong foundation, you need a project. On the other hand, if you are building a small building, you can try, based on general recommendations, to do everything yourself, including designing a reinforcement scheme.
Reinforcement scheme
The location of the reinforcement in the strip foundation in cross section is a rectangle. And there is a simple explanation for this: this scheme works best.
Reinforcement of a strip foundation with a strip height of no more than 60-70 cm
There are two main forces acting on the strip foundation: heaving forces press from below during frost, and the load from the house from above. The middle of the tape is almost not loaded. To compensate for the action of these two forces, two belts of working reinforcement are usually made: above and below. For shallow and medium deep foundations (up to 100 cm deep) this is enough. For deep belts, 3 belts are already required: too high a height requires reinforcement.
To ensure that the working fittings are in the right place, they are secured in a certain way. And they do this using thinner steel rods. They do not participate in the work, they only hold the working reinforcement in a certain position - they create a structure, which is why this type of reinforcement is called structural.
As can be seen in the strip foundation reinforcement diagram, the longitudinal reinforcement bars (working) are tied with horizontal and vertical supports. They are often made in the form of a closed loop - a clamp. It’s easier and faster to work with them, and the design is more reliable.
What fittings are needed
For strip foundations, two types of rods are used. For longitudinal ones that bear the main load, class AII or AIII is required. Moreover, the profile is necessarily ribbed: it adheres better to concrete and transfers load normally. For structural lintels, cheaper reinforcement is used: smooth first class AI, 6-8 mm thick.
Recently, fiberglass reinforcement has appeared on the market. According to manufacturers, it has better strength characteristics and is more durable. But many designers do not recommend using it in the foundations of residential buildings. According to the standards, it must be reinforced concrete. The characteristics of this material have long been known and calculated; special reinforcement profiles have been developed that ensure that metal and concrete are combined into a single monolithic structure.
How concrete will behave when paired with fiberglass, how firmly such reinforcement will adhere to concrete, how successfully this pair will resist loads - all this is unknown and has not been studied. If you want to experiment, please use fiberglass. No - take iron fittings.
Do-it-yourself calculation of strip foundation reinforcement
Any construction work is regulated by GOSTs or SNiPs. Reinforcement is no exception. It is regulated by SNiP 52-01-2003 “Concrete and reinforced concrete structures”. This document specifies the minimum amount of reinforcement required: it must be at least 0.1% of the cross-sectional area of the foundation.
Determination of reinforcement thickness
Since the strip foundation in section has the shape of a rectangle, the cross-sectional area is found by multiplying the lengths of its sides. If the tape has a depth of 80 cm and a width of 30 cm, then the area will be 80 cm * 30 cm = 2400 cm 2.
Now you need to find the total area of the reinforcement. According to SNiP it should be at least 0.1%. For this example it is 2.8 cm 2. Now, using the selection method, we will determine the diameter of the rods and their number.
Quotes from SNiP that relate to reinforcement (to enlarge the picture, right-click on it)
For example, we plan to use reinforcement with a diameter of 12 mm. Its cross-sectional area is 1.13 cm 2 (calculated using the formula for the area of a circle). It turns out that in order to provide recommendations (2.8 cm 2), we will need three rods (or they also say “threads”), since two are clearly not enough: 1.13 * 3 = 3.39 cm 2, and this is more than 2.8 cm 2, which are recommended by SNiP. But it will not be possible to divide three threads into two belts, and the load on both sides will be significant. Therefore, they stack four, laying a solid margin of safety.
In order not to bury extra money in the ground, you can try to reduce the diameter of the reinforcement: calculate it at 10 mm. The area of this rod is 0.79 cm 2. If we multiply by 4 (the minimum number of working reinforcement bars for a strip frame), we get 3.16 cm 2, which is also enough with a margin. So for this version of the strip foundation, you can use class II ribbed reinforcement with a diameter of 10 mm.
Reinforcement of a strip foundation for a cottage is carried out using rods with different types of profiles
Installation step
There are also methods and formulas for all these parameters. But for small buildings it’s simpler. According to the recommendations of the standard, the distance between horizontal branches should not be more than 40 cm. This parameter is used as a guide.
How to determine at what distance to lay reinforcement? To prevent steel from corroding, it must be embedded in concrete. The minimum distance from the edge is 5 cm. Based on this, the distance between the rods is calculated: both vertically and horizontally it is 10 cm less than the dimensions of the tape. If the width of the foundation is 45 cm, it turns out that between the two threads there will be a distance of 35 cm (45 cm - 10 cm = 35 cm), which corresponds to the standard (less than 40 cm).
The reinforcement step of a strip foundation is the distance between two longitudinal bars
If our tape is 80*30 cm, then the longitudinal reinforcement is located one from the other at a distance of 20 cm (30 cm - 10 cm). Since medium-level foundations (up to 80 cm in height) require two reinforcement belts, one belt from the other is located at a height of 70 cm (80 cm - 10 cm).
Now about how often to install jumpers. This standard is also in SNiP: the installation step of vertical and horizontal dressings should be no more than 300 mm.
All. We calculated the reinforcement of the strip foundation with our own hands. But keep in mind that neither the mass of the house nor the geological conditions were taken into account. We relied on the fact that these parameters were based on .
Corner reinforcement
In the design of a strip foundation, the weakest point is the corners and the junction of the walls. In these places loads from different walls are combined. In order for them to be successfully redistributed, the reinforcement must be properly tied. Simply connect it incorrectly: this method will not ensure load transfer. As a result, after some time, cracks will appear in the strip foundation.
The correct scheme for reinforcing corners: either bends are used - L-shaped clamps, or longitudinal threads are made 60-70 cm longer and bent around the corner
To avoid this situation, when reinforcing corners, special schemes are used: the rod is bent from one side to the other. This “overlap” should be at least 60-70 cm. If the length of the longitudinal rod is not enough to bend, use L-shaped clamps with sides also at least 60-70 cm. Schemes of their location and fastening of the reinforcement are shown in the photo below.
The abutments of piers are reinforced using the same principle. It is also advisable to take the reinforcement with a reserve and bend it. It is also possible to use L-shaped clamps.
Reinforcement diagram for adjacent walls in a strip foundation (to enlarge the picture, right-click on it)
Please note: in both cases, in the corners, the installation step of the transverse jumpers is reduced by half. In these places they already become workers - they participate in the redistribution of the load.
Reinforcement of the base of a strip foundation
On soils with not very high bearing capacity, on heaving soils or under heavy houses, strip foundations are often made with a sole. It transfers the load to a larger area, which gives greater stability to the foundation and reduces the amount of subsidence.
To prevent the sole from falling apart under pressure, it also needs to be reinforced. The figure shows two options: one and two belts of longitudinal reinforcement. If the soils are complex, with a strong tendency to winter baking, then two belts can be laid. For normal and medium-heaving soils, one is enough.
Reinforcement rods laid lengthwise are working. They, as for the tape, are taken in second or third class. They are located at a distance of 200-300 mm from each other. They are connected using short pieces of rod.
Two methods of reinforcing the base of a strip foundation: on the left for foundations with normal bearing capacity, on the right for not very reliable soils
If the sole is not wide (rigid design), then the transverse segments are structural and do not participate in load distribution. Then they are made with a diameter of 6-8 mm, bent at the ends so that they cover the outer rods. They are tied to everyone using a binding wire.
If the sole is wide (flexible), the transverse reinforcement in the sole is also working. She resists the soil’s attempts to “collapse” her. Therefore, in this version, the soles use ribbed reinforcement of the same diameter and class as the longitudinal one.
How much rod do you need?
Having developed a strip foundation reinforcement scheme, you know how many longitudinal elements you need. They are laid around the entire perimeter and under the walls. The length of the tape will be the length of one reinforcement rod. By multiplying it by the number of threads, you get the required length of the working reinforcement. Then add 20% to the resulting figure - a margin for joints and overlaps. This is how much in meters you will need working reinforcement.
Now you need to calculate the amount of structural reinforcement. Calculate how many crossbars there should be: divide the length of the tape by the installation pitch (300 mm or 0.3 m, if you follow the recommendations of SNiP). Then you calculate how much it takes to make one lintel (add the width of the reinforcement cage with the height and double it). Multiply the resulting figure by the number of jumpers. You also add 20% to the result (for connections). This will be the amount of structural reinforcement to reinforce the strip foundation.
Using a similar principle, you calculate the amount needed to reinforce the sole. Putting everything together, you will find out how much reinforcement is needed for the foundation.
Technologies for assembling reinforcement for strip foundations
Do-it-yourself reinforcement of the strip foundation begins after installation. There are two options:
Both options are imperfect and everyone decides how it will be easier for him. When working directly in a trench, you need to know the procedure:
- The longitudinal rods of the lower reinforced belt are laid first. They need to be raised 5 cm from the edge of the concrete. It is better to use special legs for this, but pieces of bricks are popular among developers. The reinforcement is also 5 cm away from the formwork walls.
- Using transverse pieces of structural reinforcement or molded contours, they are fixed at the required distance using a knitting wire and a hook or a knitting gun.
- Then there are two options:
- If contours formed in the form of rectangles were used, the upper belt is immediately tied to them at the top.
- If during installation you use cut pieces for crossbars and vertical posts, then the next step is tying up the vertical posts. After they are all tied, a second belt of longitudinal reinforcement is tied.
There is another technology for reinforcing strip foundations. The frame turns out to be rigid, but there is a large consumption of rods for the vertical posts: they are driven into the ground.
The second technology for reinforcing a strip foundation is to first drive in vertical posts, tie longitudinal threads to them, and then connect everything with transverse ones
- First, vertical posts are driven in at the corners of the tape and at the junction of horizontal rods. The racks should have a large diameter of 16-20 mm. They are placed at a distance of at least 5 cm from the edge of the formwork, checking horizontal and vertical, and driven into the ground 2 meters.
- Then vertical rods of the calculated diameter are driven in. We determined the installation pitch: 300 mm, in the corners and at the junctions of the walls it is half as much - 150 mm.
- The longitudinal threads of the lower reinforcement belt are tied to the posts.
- At the intersection of the racks and longitudinal reinforcements, horizontal jumpers are tied.
- The upper reinforcement belt is tied, which is located 5-7 cm below the upper surface of the concrete.
- Horizontal jumpers are tied.
It is most convenient and quick to make a reinforcing belt using pre-formed contours. The rod is bent to form a rectangle with the specified parameters. The whole problem is that they need to be made identical, with minimal deviations. And a large number of them are required. But then the work in the trench moves faster.
As you can see, reinforcing a strip foundation is a lengthy and not the easiest process. But you can cope even alone, without helpers. It will take a lot of time, though. It’s easier to work with two or three people: both carry the rods and set them out.
